As Hurricane Irene moved north, with its winds and torrents of rain, it caused tornadoes in at least four states, and officials warned they could be as dangerous as the hurricane itself.

Tornadoes were reported in New Jersey, Virginia, Maryland and Delaware.

Delaware's Gov. Jack Markell said he believed a tornado tore through 17 homes near Lewes, Del., and off Rehoboth Beach, the National Weather Service reported a waterspout -- essentially, a tornado out at sea. There were no reports of major injuries.

At least five homes in the Sandbridge area of Virginia Beach, Va., sustained major damage from a tornado, said Mary Hancock, a spokeswoman for the city, but the area had already been evacuated and there were no injuries reported. Several other homes in the area were less seriously damaged.

"The category of hurricane doesn't explain all the risk," Federal Emergency Management Agency Administrator Craig Fugate said. "Rainfall and tornadoes are not tied to the category of a storm ... some of the most devastating floods have been from tropical storms."

What do tornadoes have to do with hurricanes? A hurricane is a giant vortex of low presure sending wind and rain whipping around -- but also drawing air in from other, dryer areas to fill the vacuum created by the storm. When cooler, dryer air collides with the muggy air of the hurricane, the conditions are ripe for funnel clouds to spin off in almost any direction.

While the tornadoes are much smaller than the hurricane itself, they can be considerably more intense. Their strength -- and the amount of damage they can cause -- is not related to the strength of the hurricane.

Americans most often hear about tornadoes in the Midwest and South, especially in the spring as cold air from Canada is pushed back by the growing warmth in the Gulf of Mexico. That formula makes for the largest number of tornadoes on earth. But meteorologists say they can happen anywhere on the planet -- and in a hurricane, all bets are off.

In fact, most of the deaths from hurricanes historically do not come from the hurricane itself, but from its side effects -- flooding, downed trees and power lines, and tornadoes. The first death from Irene in North Carolina came even before the storm arrived; it was a man who reportedly had a heart attack while boarding up his house.

NOAA says the most dangerous time is usually after the storm is gone, and people mistakenly think it is safe to wade through the water it has left.

"I need to stress when it stops raining, doesn't mean that it will stop flooding," Pennsylvania Gov. Tom Corbett said.

People in Massachusetts were on guard, too. The area around Springfield, Mass., was badly damaged by a tornado on June 1, and the area is likely to be right in the path of Irene's remnants Sunday as the storm crosses New England.

More than 300,00 people evacuated low-lying parts of New York City, which was told to expect the first of Irene's winds on Saturday night and into Sunday.

"New Yorkers like to think we are tough," Gov Andrew Cuomo said. "We are smart enough to know we don't mess with Mother Nature."